The present invention relates to surface acoustic wave devices, and more particularly to surface acoustic wave devices employing two transducers which are acoustically coupled through use of a multi-strip coupler.
Surface acoustic wave (SAW) devices include a substrate of piezoelectric material, such as lithium niobate, upon which a metallized pattern is formed. One part of the pattern represents an input transducer and another part represents an output transducer. The input transducer transforms an applied electrical signal into an acoustic wave propagating along the surface of the substrate material. The output transducer is disposed in the path of the acoustic wave, and transforms it back into an electrical output signal, usually having somewhat different characteristics than the input signal.
Some conventional design surface acoustic wave devices include input and output transducers which are disposed in different acoustic tracks on the piezoelectric substrate. Acoustic energy is coupled between the two acoustic tracks by use of a third metallized pattern, known as a multi-strip coupler, which extends across both tracks. The multi-strip coupler is largely insensitive to the bulk acoustic waves which are incidentally generated by the input transducer. Because of this, bulk acoustic waves remain within the acoustic track of the input transducer and do not interfere with the operation of the output transducer.
The transfer characteristic of a surface acoustic wave device can be selected by careful design of the input and output transducers. The transfer characteristic of a surface acoustic wave device can, for example, be designed to be frequency selective, whereby the device acts as a filter. Surface acoustic wave devices are quite popular as filters, since they can be designed to exhibit sharp cut-off frequencies and flat pass band response characteristics. For this reason surface acoustic wave devices have found use, for example, in television transmitters and receivers.
An ideal bandpass filter would have a flat frequency response (amplitude and phase) across the pass band, and zero response to frequencies outside of the pass band. Performance of SAW filters approach that of an ideal filter in many respects. Nonetheless, there is some residual deviation from a flat transfer characteristic across the pass band of the filter. The ripple in the pass band transfer characteristic of the SAW filter is undesirable. When the SAW filter is used in a TV transmitter as a vestigial sideband (VSB) filter, for example, the residual frequency response ripple introduces some distortion into the video signal, thereby degrading the video image subsequently presented. It would be desirable to provide a SAW filter which exhibited reduced ripple in the frequency response characteristic of the filter across the pass band thereof.